The present invention relates to a device for feeding the fabric being sewn by sewing machines.
As is known, in sewing machines, the fabrics being sewn are fed by feed dog teeth which are arranged in the region of the needle plate and are movable so as to feed new fabric continuously between the needle plate and the pressure shoe or pressure foot.
These teeth are operated by special mechanisms which form the devices for feeding the fabrics being sewn.
These devices are of fundamental importance in sewing machines, since proper functioning of the sewing machines and the possibility of making stitches of varying length, which are more or less taut, depend on the correct operation of the said devices and on the possibility of varying the feed rate of the fabric, as required.
Numerous feed devices have already been devised and produced: reference is made to the feed devices already produced by the same Applicant and the feed device described in Italian Pat. No. 856,128.
Of these devices, the most efficient and important ones for industrial sewing machines are those which involve the use of two sets of feed teeth: a set of main teeth and a set of differential feed dog teeth arranged in succession and movable cyclically in a synchronized manner.
The main feed dog teeth, which are also called stitch feed dog teeth, determine substantially the amount of fabric which must pass beneath the pressure shoe.
The differential or secondary feed dog teeth, on the other hand, wrinkle or pucker the fabric when they move by amounts greater than those of the main teeth, or may tension the fabric when, on the contrary, they move by smaller amounts.
Puckering is generally performed for esthetic reasons, whereas tensioning may be necessary when an elastic fabric is being sewn and it is required that the stitches should not restrict the elasticity of the fabric itself. In this case, tensioning the fabric during sewing has the effect that, once the fabric has been released, the sewing thread is of sufficient length and the stitches are not taut.
So that the differential feed dog teeth are able to move by amounts different from those of the main feed dog teeth, while remaining in synchronism with the latter, existing feed devices comprise considerably complex kinematic mechanisms which extend from at least three drive shafts, with all of the movements produced by a main rotating shaft. The feed dog oscillations of the teeth in the vertical direction are obtained by means of cams mounted on rotating shafts, whereas the oscillations in the horizontal direction are obtained by means of connecting rod/crank arrangements. The various devices contain a large number of components in order that all the oscillations may be performed, without each of these oscillations generating obstruction to the others. Moreover, it appears to be complicated and in many cases very difficult to adjust the movements of the differential feed dog teeth so as to obtain movements of the same which are greater or less than those of the main feed dog teeth. This operative characteristic of known feeling devices appears restrictive of the use thereof.